Hats are perfect for lumpy bumpy yarn. Way back before I'd even dreamt of keeping sheep or learning to spin, my aunt presented my family with handspun and hand knitted woolly hats she had made herself from her Soays. The yarn was barely spun, which made it wonderfully soft and squashy, and the lumpy bumpy effect was wonderful - we all loved those hats and wore them for years.
Something else I use lumpy bumpy for is as a highlight within another article - say subtle stripes across a jumper, or to give some texture to a shawl. Mind you, I'm quite the opposite of a perfectionist, so I have never tried to spin to a set gauge - what suits the fibre I'm spinning is how the yarn comes out and I then invent a pattern to fit.
This explains why I'm still struggling away with my Heb+silk shawl. It's not a simple triangle or oblong, but has to be shaped rather precisely. Whenever I try to work out exactly where to do my increases, my brain suddenly turns to mush. However, I think I sorted it when I was sleeping last night